Transform data using R (SQL Server and RevoScaleR tutorial)

APPLIES TO: yesSQL Server noAzure SQL Database noAzure SQL Data Warehouse noParallel Data Warehouse

This lesson is part of the RevoScaleR tutorial on how to use RevoScaleR functions with SQL Server.

In this lesson, learn about the RevoScaleR functions for transforming data at various stages of your analysis.

  • Use rxDataStep to create and transform a data subset
  • Use rxImport to transform in-transit data to or from an XDF file or an in-memory data frame during import

Although not specifically for data movement, the functions rxSummary, rxCube, rxLinMod, and rxLogit all support data transformations.

Use rxDataStep to transform variables

The rxDataStep function processes data one chunk at a time, reading from one data source and writing to another. You can specify the columns to transform, the transformations to load, and so forth.

To make this example interesting, let's use a function from another R package to transform the data. The boot package is one of the "recommended" packages, meaning that boot is included with every distribution of R, but is not loaded automatically on start-up. Therefore, the package should already be available on the SQL Server instance configured for R integration.

From the boot package, use the function inv.logit, which computes the inverse of a logit. That is, the inv.logit function converts a logit back to a probability on the [0,1] scale.

Tip

Another way to get predictions in this scale would be to set the type parameter to response in the original call to rxPredict.

  1. Start by creating a data source to hold the data destined for the table, ccScoreOutput.

    sqlOutScoreDS <- RxSqlServerData( table =  "ccScoreOutput",  connectionString = sqlConnString, rowsPerRead = sqlRowsPerRead )
    
  2. Add another data source to hold the data for the table ccScoreOutput2.

    sqlOutScoreDS2 <- RxSqlServerData( table =  "ccScoreOutput2",  connectionString = sqlConnString, rowsPerRead = sqlRowsPerRead )
    

    In the new table, store all the variables from the previous ccScoreOutput table, plus the newly created variable.

  3. Set the compute context to the SQL Server instance.

    rxSetComputeContext(sqlCompute)
    
  4. Use the function rxSqlServerTableExists to check whether the output table ccScoreOutput2 already exists; and if so, use the function rxSqlServerDropTable to delete the table.

    if (rxSqlServerTableExists("ccScoreOutput2"))     rxSqlServerDropTable("ccScoreOutput2")
    
  5. Call the rxDataStep function, and specify the desired transforms in a list.

    rxDataStep(inData = sqlOutScoreDS,
        outFile = sqlOutScoreDS2,
        transforms = list(ccFraudProb = inv.logit(ccFraudLogitScore)),
        transformPackages = "boot",
        overwrite = TRUE)
    

    When you define the transformations that are applied to each column, you can also specify any additional R packages that are needed to perform the transformations. For more information about the types of transformations that you can perform, see How to transform and subset data using RevoScaleR.

  6. Call rxGetVarInfo to view a summary of the variables in the new data set.

rxGetVarInfo(sqlOutScoreDS2)

Results

Var 1: ccFraudLogitScore, Type: numeric
Var 2: state, Type: character
Var 3: gender, Type: character
Var 4: cardholder, Type: character
Var 5: balance, Type: integer
Var 6: numTrans, Type: integer
Var 7: numIntlTrans, Type: integer
Var 8: creditLine, Type: integer
Var 9: ccFraudProb, Type: numeric

The original logit scores are preserved, but a new column, ccFraudProb, has been added, in which the logit scores are represented as values between 0 and 1.

Notice that the factor variables have been written to the table ccScoreOutput2 as character data. To use them as factors in subsequent analyses, use the parameter colInfo to specify the levels.

Next steps